This invention relates to a method of controlling a starter system for a heat engine, of the type comprising two starters so arranged that their pinions act in parallel on the starter crown of the engine after the ignition key has been actuated, each starter comprising an electric motor for driving its pinion and a power interrupter connected in the power circuit of the electric motor and controlled by a power supply to a coil.
The invention further relates to apparatus for performing the above methods.
Methods and apparatus of the above types are known at this time. In this connection, in some installations it is preferable to make use of two small starters in parallel rather than one single large starter. Thus, for very high capacity engines, the use of a pair of small mass produced starters instead of one single large starter made in smaller quantities may be more inexpensive.
Reference is here made to FIG. 1 of the accompanying drawings, described later herein under the heading xe2x80x9cBrief Description of the Drawingsxe2x80x9d.
The starter system shown diagrammatically in FIG. 1 includes two small starters 1 and 2, having respective pinions 3, 3xe2x80x2 which actuate in parallel the starter crown 4 of the heat engine (not shown). The electric motors of the starters 1 and 2 are indicated at 5 and 5xe2x80x2 respectively, and their power interrupters, or xe2x80x9ccontactsxe2x80x9d, at 6 and 6xe2x80x2 respectively. Each of these interrupters is controlled by the moving core of an electromagnet which includes a starting coil, denoted 7 for the starter 1 and 7xe2x80x2 for the starter 2, and a running coil, denoted 8 for the starter 1 and 8xe2x80x2 for the starter 2. For more details about the construction of a starter of this type, reference may for example be made to French published patent specification FR 2 749 451A, given that such a starter may include only one coil in the manner described in French published patent specification FR 2 795 884A, in which the starter is again described.
Both starters are supplied with electrical energy from a battery 9 producing a voltage U, once the ignition has been switched on by the key 10. In such a starter system, each starter contributes its own power.
It has been found that starter systems of the kind shown in FIG. 1 are susceptible to serious variations in operating behaviour, such as to give rise to rapid deterioration of the starters in order to assist understanding of these behavioural eccentricities, the principle of operation of a starter of the twin-starter type, typified by the starters 1 and 2 in FIG. 1, will be briefly mentioned with reference to FIG. 2.
In FIG. 2, current is plotted against time T on the abscissa. It shows in the form of a full line the characteristic curve for the current taken by the starter, and, in broken lines, the voltage U available at the battery 9. On closure of the ignition key 10 at the moment t0, the running coil 8 and the starting coil 7 are simultaneously energised. The current IC1 absorbed by the coils is then generally between 40 and 60 amps for a system supplied at a nominal 12 volts. The battery voltage U, equal to U0 before the ignition key is closed, falls slightly because of the provision of the current IC1. Because of the magnetic effects of the coils, the current IC1 displaces the pinion 3 towards the crown 4 through the interposed moving coil of the starter contactor. At the instant t1, the power interrupter 6 closes the power supply circuit of the electric motor and thus causes the current I1 to flow, which causes a current peak IC1+I1, taken by the motor, to occur at the instant T2. This current IC1+I1 then reduces as the motor picks up speed. The starting current produces a very deep trough, which may reach 6 to 7 volts, in the voltage U. With effect from the instant t1, the contactor is supplied with power only through its single running coil 8. Its consumption falls back to a value of 8 to 10 amps. This results in a large fall in the value of the magnetic forces. However, this value does remain large enough to enable the magnetic core to finish its travel and to ensure, even at the instant t2, that the moving core is held magnetically against the fixed part of the contactor.
Reference is now made to FIG. 3 of the drawings, to explain the various behavioural anomalies which can occur due to starter operation as described above, where two conventional starters are combined in the manner shown in FIG. 1. FIG. 3 shows the characteristic curves of the currents IC1 and IC2 which are absorbed by the contactors of the two starters, the currents I1, I2 with which the electric motors 5, 5xe2x80x2 are supplied, and the battery voltage U, all as a function of time t. When the ignition key 10 is closed at the instant t0, the power interrupter 6 of the starter 1 closes at the instant t1. The interrupter 6xe2x80x2 of the starter 2 closes after a slight time delay dt of a few milliseconds, because of variations in the characteristics of the different starters. The total current IC1+I1 absorbed by the starter 1 rises sharply from the instant t1, and then slows and reduces at A because of the second starter 2, which consumes the total current in accordance with the curve I2+IC2. This gives rise to a very large drop in the battery voltage U. One of the two starters reaches its unengaged voltage threshold, this being generally the second starter because its moving core may not have finished its travel. The residual air gap existing at this instant reduces the magnetic forces. Since the return force is higher than the motive forces of the electromagnet, the moving core of this starter therefore returns to its rest position. The power interrupter 6xe2x80x2 opens, and the current intensity drops from B to C. The battery, relieved of the consumption of this starter, sees an increase in its voltage which enables the power interrupter 6 of the starter 1 to stay closed. The starting peak of the latter once again rises, from A to D. This starter then begins to turn, and the intensity of the current I1+IC1 increases to the point E. In conjunction with this, the battery voltage U increases.
During this time, the power interrupter of the second starter 2, which has been open since the point C, once again permits simultaneous supply of power to the starting and holding coils. The magnetic forces are now increasing very sharply, especially since the battery voltage U is once again rising. The power interrupter of the second starter once again closes and causes a second peak to occur in the intensity of the current I2+IC2, at the point F. This causes the interrupter once again to open, for the same reasons as before. However, in the meantime, the speed of the motor 1 continues to increase, and therefore the intensity of the current it takes continues to diminish. On the third closing event at H, the sum of the currents absorbed by the two starters is low enough for there no longer to be any re-opening. Starting of the heat engine can then take place normally.
It can easily be understood that, when such starting conditions occur, the starters undergo sharp variations in operating mode, whereby severe forces are applied to them both from the mechanical point of view (by virtue of impulses on the shaft line, risk of disengagement of the crown, and so on), and from the electrical point of view (for example by virtue of sparking, and arcs on the commutators and contactor contacts when current peaks occur).
Apart from the undesirable effects described above, other disadvantageous phenomena can occur. These depend on the type of characteristics of the starters used. It can happen that the time difference dt mentioned above is very large if the first starter has the time to gather a high speed before the pinion of the second starter comes into contact with the starter crown. The velocity of the crown is then too high to enable that pinion to be able to engage. This gives rise to wear and rapid damage to both pinions and the crown. In addition, the moving core of the second starter may recoil by an amount high enough to disengage its pinion from the crown. During the second engagement at the time t2, the first starter drives the crown at a speed too high for the pinion of the second starter to be able to re-engage in the crown. As before, high wear and rapid damage to the pinion and crown will ensue.
In order to overcome these drawbacks, it has been proposed to provide relay units in which the power circuit for the starters is in series with a relay which will only close at the end of a certain time delay after the ignition is switched on. This enables the contactor to close fully and to be in a stable position at the instant when current peaks occur in the two starters.
It has also been proposed to insert in the power circuit of the two starters a resistance to reduce the magnitude of the current peak. This ballast resistance is short circuited by a relay when the current intensity has once again dropped sufficiently low, or when, with the aid of a time delay, a predetermined time has elapsed.
Such units do however have certain drawbacks, namely that they are expensive and bulky, and call for additional wiring of the vehicle which is more complicated and therefore more costly. In addition, control of these relay devices requires a high electric current, and as a result they cannot be operated directly through the ignition key. This then calls for an additional auxiliary relay.
An object of the present invention is to propose a method and apparatus for starting, which mitigate, not only the disadvantages first described above, but also the disadvantages of known apparatus using relays.
In order to achieve the above object, the method of controlling a heat engine starter according to the invention is characterised in that the closure of the power interrupter of one of the starters is delayed with respect to the other by a time delay the magnitude of which is chosen to be such as to avoid reopening of one of the interrupters that could be caused by the voltage drop produced by the closing of the power interrupter of the said other starter.
According to another feature of the invention, the closing of the said power interrupter is delayed by slowing down the displacement of the control core of that interrupter towards its closure position.
According to a further feature of the invention, the displacement of the core is slowed down by causing the supply current to that coil of the starter for which closing is retarded to be weaker than the current with which the coil of the other starter is supplied.
According to yet another feature of the invention, the intensity of the supply current to the coil of the starter for which closing is retarded is increased by a value which enables the core to pursue its travel to closure of the power interrupter during the drop in voltage produced by closing of the interrupter of the first starter.
According to a still further feature of the invention, the supply current of the retarded starter is increased on closing of the interrupter of the latter during a time interval such as to ensure engagement of its pinion in the starter crown.
According to another feature of the invention, the displacements of the cores of one of the starters and also the other are controlled, and the displacement of the core of the other starter is commenced at an instant of time between commencement of the movement of the core of the first starter and closure of the power interrupter of the latter.
According to a further feature of the invention, the supply current of one starter is controlled by command of an interrupter connected in the supply circuit to the coil of the starter.
According to yet another feature of the invention, where the interrupter is actuated by a pulse width modulated signal, the said supply current is controlled by causing the cyclic ratio of the latter to vary.
According to a still further feature of the invention, the cyclic ratio is appropriately chosen during various phases of the starting operation, in particular during the phase in which the first electric starter motor is itself started, the phase in which the second starter motor is started, and the phase in which the starter system drives the heat engine after the phase of complete engagement of the pinions of the two starters.
The apparatus for controlling starting of a heat engine, for performing the method according to the invention, is characterised in that at least one of the starters is equipped with an electronic control unit for controlling the starting of its electric starter motor.
According to another feature of the invention, an electronic interrupter is connected in the power circuit for the coil of the starter motor.
According to yet another feature of the invention, the electronic control unit produces a said pulse width modulated control signal for controlling the interrupter.
According to yet another feature of the invention, both starters are equipped with a said electronic control unit.